### How to correctly select silicon controlled components for induction melting furnace

How to correctly select silicon controlled components for induction melting furnace

The correct selection of power electronic devices such as thyristors and rectifiers is of great significance to ensure the reliability of the induction melting furnace and reduce the cost of the induction melting furnace. The selection of components should comprehensively consider factors such as its use environment, cooling method, circuit type, load properties, etc., and take into account economy under the condition of ensuring that the parameters of the selected components have margins.

Since the application fields of power electronic devices are very wide, and the specific application forms are various, the following only describes the selection of thyristor components in rectifier circuits and single-phase intermediate frequency inverter circuits.

1 Rectifier circuit device selection

Power frequency rectification is one of the most commonly used fields of SCR components. The component selection mainly considers its rated voltage and rated current.

(1) The forward and reverse peak voltages VDRM and VRRM of the thyristor device :

It should be 2-3 times of the maximum peak voltage UM that the component actually bears , that is, VDRM/RRM=(2-3)UM . The UM values corresponding to various rectification circuits are shown in Table 1 .

(2) Rated on-state current IT (AV) of the thyristor device :

The IT (AV) value of the thyristor refers to the average value of the power frequency sine half-wave, and its corresponding effective ITRMS=1.57IT(AV) . In order to prevent the component from being damaged by overheating during operation, the actual effective value flowing through the component should be equal to 1.57IT(AV) after being multiplied by a safety factor of 1.5-2 . Assuming that the average load current of the rectifier circuit is Id and the effective value of the current flowing through each device is KId , the rated on-state current of the selected device should be :

IT(AV)=(1.5-2)KId/1.57=Kfd*Id

Kfd is the calculation coefficient. For the control angle α= 0O , the Kfd values under various rectifier circuits are shown in Table 1 .

Table 1: The maximum peak voltage UM of the rectifier device and the calculation coefficient Kfd of the average on-state current

 Rectifier circuit Single phase half wave Single double half wave Single bridge Three phase half wave Three-phase bridge With balanced reactor Double reverse star UM U2 U2 U2 U2 U2 U2 Inductive load 0.45 0.45 0.45 0.368 0.368 0.184

Note : U2 is the effective value of the secondary phase voltage of the main loop transformer ; the single half-wave inductive load circuit has a freewheeling diode.

When selecting the component IT (AV) value, the heat dissipation mode of the component should also be considered. In general, the rated current value of the same component of air cooling is lower than that of water cooling ; in the case of natural cooling, the rated current of the component should be reduced to one-third of the standard cooling condition.